The present invention particularly relates to an apparatus for and a method of measuring period jitter that are applied to a measurement of jitter of, for example, a microprocessor clock or another periodic signal.
A time interval analyzer and/or an oscilloscope have conventionally been used for the measurement of period jitter. The method of these apparatus is called Zero-crossing Method, in which, as shown in FIG. 1, a clock signal (a signal under measurement) x(t) from, for example, a PLL (Phase-Locked Loop) under test 11 is supplied to a time interval analyzer 12. Regarding a signal under measurement x(t), a next rising edge following one rising edge fluctuates against the preceding rising edge as indicated by dotted lines. That is, a time interval Tp between the two rising edges, namely a period fluctuates. In the Zero-crossing Method, a time interval between zero-crossings (period) of the signal under measurement is measured, and a fluctuation of period is measured by a histogram analysis. The histogram is displayed, for example as shown in FIG. 2, by the time interval analyzer 12. A time interval analyzer is described in, for example, “Phase Digitizing Sharpens Timing Measurements” by D. Chu, IEEE Spectrum, pp. 28–32, 1988, and “A Method of Serial Data Jitter Analysis Using One-Shot Time Interval Measurements” by J. Wilstrup, Proceedings of IEEE International Test Conference, pp. 819–823, 1998.
Tektronix, Inc. and LeCroy Co. have recently been providing digital oscilloscopes each being able to measure a jitter using an interpolation method. In this jitter measurement method using the interpolation method (interpolation-based jitter measurement method), an interval between data having signal values close to a zero-crossing out of measured data of a sampled signal under measurement is interpolated to estimate a timing of zero-crossing. That is, in order to estimate a fluctuation of period, a time interval between zero-crossings (period) is estimated using a data interpolation with a small error.
That is, as shown in FIG. 3, a signal under measurement x(t) from the PLL under test 11 is inputted to a digital oscilloscope 14. In the digital oscilloscope 14, as shown in FIG. 4, the inputted signal under measurement x(t) is converted into a digital data sequence by an analog-to-digital converter 15. A data-interpolation is applied to an interval between data having signal values close to a zero-crossing in the digital data sequence by an interpolator 16. With respect to the data-interpolated digital data sequence, a time interval between zero-crossings is measured by a period estimator 17. A histogram of the measured values is displayed by a histogram estimator 18. In addition, a root-mean-square value and a peak-to-peak value of fluctuations of the measured time intervals are obtained by an RMS & Peak-to-Peak Detector 19. For example, in the case in which a signal under measurement x(t) has a waveform shown in FIG. 5A, its period jitters are measured as shown in FIG. 5B.
On the other hand, inventors of the present invention have proposed a method of measuring a jitter as described below in an article entitled “Extraction of Peak-to-Peak and RMS Sinusoidal Jitter Using an Analytic Signal Method” by T. J. Yamaguchi, M. Soma, M. Ishida, and T. Ohmi, Proceedings of 18th IEEE VLSI Test Symposium, pp. 395–402, 2000. That is, as shown in FIG. 6, an analog clock waveform from a PLL (Phase locked loop) circuit under test 11 is converted into a digital clock signal xc(t) by an analog-to-digital converter 22, and the digital clock signal xc(t) is supplied to a Hilbert pair generator 23 acting as an analytic signal transforming part, where the digital clock signal xc(t) is transformed into an analytic signal zc(t).
In the Hilbert pair generator 23, signal components around a fundamental frequency of the clock signal under measurement xc(t) are extracted by a band-pass filter (not shown), and the extracted signal components are inputted to a Hilbert transformer 25, where the signal components are Hilbert-transformed. An analytic signal zc(t) having the signal component not Hilbert-transformed and the signal component Hilbert-transformed as a real part and an imaginary part of a complex number, respectively is obtained from the Hilbert pair generator 23.
An instantaneous phase Θ(t) of the clock signal xc(t) is estimated by an instantaneous phase estimator 26 from the analytic signal zc(t). A linear phase is removed by a linear phase remover 27 from this instantaneous phase Θ(t), and an instantaneous phase noise waveform Δφ(t) is obtained.
This instantaneous phase noise Δφ(t) is sampled by a zero-crossing sampler 28, and the sampled instantaneous phase noise is inputted to a peak-to-peak detector 32 as a timing jitter sequence Δφ[n], where a difference between the maximum peak value max (Δφ[k]) and the minimum peak value min (Δφ[k]) of the Δφ[n](=Δφ(nT)) is calculated to obtain a peak value (peak-to-peak value) Δφpp of timing jitter as follows.
      Δϕ    pp    =                    max        k            ⁢              (                  Δϕ          ⁡                      [            k            ]                          )              -                  min        k            ⁢              (                  Δϕ          ⁡                      [            k            ]                          )            In addition, the timing jitter sequence Δφ[n] is also inputted to a root-mean-square detector 33, where a root-mean-square (RMS) value of the timing jitter sequence Δφ[n] is calculated using following equation to obtain a root-mean-square value ΔφRMS of timing jitter.
      Δϕ    RMS    =                    1        N            ⁢                        ∑                      k            =            0                                N            -            1                          ⁢                              Δϕ            2                    ⁡                      [            n            ]                              
This method is referred to as the Δφ method, since a peak value of timing jitter (peak-to-peak value) and a root-mean-square value of timing jitter are obtained as described above from the instantaneous phase noise waveform Δφ(t).
According to the Δφ method, a timing jitter can be measured at high speed with relatively high accuracy.
Since the jitter measurement method in the time interval analyzer system includes an intermediate dead-time during which no measurement can be performed after each one of periodic measurements is performed, there is a problem that it takes a long time to acquire a number of data that are required for a histogram analysis.
In addition, in a jitter measurement method in which a wide-band oscilloscope and an interpolation method are combined, there is a problem that a jitter is overestimated (overestimation). That is, the measured jitter values in this method are not compatible with the values measured by the time interval analyzer method. For example, a jitter measurement result by the time interval analyzer and a jitter measurement result by the interpolation method for a 400 MHz clock signal are shown in FIGS. 7A and 7B, respectively so that those measurement results can be compared with each other. According to those figures, the measured value by the time interval analyzer is 7.72 ps (RMS), while the measured value by the interpolation method is 8.47 ps (RMS), and this value is overestimated. In addition, the jitter measurement method by the interpolation method requires a long measurement time.
In addition, in the conventional Δφ method, a zero-crossing point of a signal under measurement is approximated by a sampling point closest to each zero-crossing point (this point is referred to as approximated zero-crossing points). Therefore, there is a possibility that an amount of error in jitter measurement is increased when an over-sampling rate in the signal digitization process is small.
It is an object of the present invention to provide a jitter measurement apparatus and its method that can estimate a jitter value compatible with a jitter value measured by the conventional time interval analyzer method.